1. Field of the Invention
The present invention relates to a direct sequence-spread spectrum (DS-SS) communication method and a DS-SS communication apparatus that are used for radio communication.
2. Description of the Related Art
As a method for implementing a high-rate DS-SS communication, a carrier frequency offset-spread spectrum method (which will be referred to as a CFO-SS method, hereinbelow) is disclosed in Japanese Patent Application Laid-Open No. 10-107771 (xe2x80x9cSpread-Spectrum Communication Methodxe2x80x9d; by Ishikawa, Shinonaga, and Kobayashi). The disclosed method is characterized as described below. Either a single or a plurality of communication stations share the identical pseudo-random code series, and thereby perform an independent DS-SS modulation for each of multiple information data streams. Carrier frequencies separated from each other by an integer multiple in units of the frequency amount that is defined by the information baud rate are used, and radio communication is thereby performed for a plurality of the signals each for which a DS-SS modulation has been performed. Thereby, communication can be implemented without interwave interference being caused among a plurality of spread spectrum signals.
FIG. 8 is a conceptual view of the CFO-SS method. According to the CFO-SS method, an information signal to be transmitted is divided into parallel information signals. In FIG. 8, there is shown an example where an information signal is separated into n parallel signals. In the figure, the denotations of the reference numbers are as follows: 70-1 to 70-n: Each denotes a divisional information signal obtained from an original information signal; 71-1 to 71-n: Each denotes a spreading modulator for performing spectrum-spreading for corresponding one of the information signals 70-1 to 70-n according to corresponding one of PN codes 72-1 to 72-n; 72-1 to 72-n: Each denotes the PN code, as mentioned above; 73-1 to 73-n: Each denotes a modulator for performing modulation of a corresponding baseband signal obtained by corresponding one of the aforementioned spreading modulators 71-1 to 71-n, the modulation being performed according to corresponding one of local oscillators 74-1 to 74-n; 74-1 to 74-n: Each denotes the local oscillator, as mentioned above; 75-1 to 75-n: Each denotes a bandpass filter for extracting a frequency component required for transmission from corresponding one of modulated signals; 76: Denotes a synthesizer for equivalently indicating that spread spectrum signals of the individual parallel information signals 70-1 to 70-n are multiplexed and transmitted according to individually unique carrier frequencies; 77: Denotes a bandpass filter for extracting only a frequency component required for demodulation from each of received signals; 78-1 to 78-n: Each denotes a frequency converter for converting an high-frequency signal output from the bandpass filter 77 to a baseband signal by using a signal from corresponding one of local oscillators 79-1 to 79-n; 79-1 to 79-n: Each denotes the local oscillator, as described above; 80-1 to 80-n: Each denotes a demodulator for detecting an information signal from a band-limited received signal; and 81-1 to 81-n: Each denotes an information signal detected by corresponding one of the aforementioned demodulators 80-1 to 80-n.
Thus, the CFO-SS method is characterized such that the information signal from the individual communication station is separated into a set of parallel information signals, and they are modulated according to the DS-SS modulation method using the identical pseudo-random code series. As a result, as shown in FIG. 9, the carrier frequencies are modulated so as to be separated from each other, frequency bands occupied by the spread spectrum signal waves are set so as to overlap with each other, and then the waves are thereby transmitted. At this time, when carrier frequencies separated from each other in units of an integer multiple of the frequency amount R that is defined by the information baud rate are used for each spread spectrum signal wave, a receiver can extract desired waves of which the carrier frequencies are known. This allows desired information to be optimally received and demodulated without interference from other signal waves.
However, the CFO-SS method has problems. The spread spectrum signal wave separated by the frequency amount R is permitted to exist at either one side or two sides of a desired spread spectrum signal wave. In this case, a quality level of transmission lines is reduced because the line is interfered by influences of the adjacent signal wave or waves. The influences are caused by, for example, multipath fading characteristics, band-limitation characteristics with respect to transmission signals, and timing offsets among the spread spectrum signal. To solve these problems, a communication method was proposed as is disclosed in Japanese Patent Application Laid-Open No. 10-135870 (xe2x80x9cSpread Spectrum Communication Systemxe2x80x9d; by Ishikawa, Shinonaga, and Kobayashi). The disclosure can be summarized such that, as shown in FIG. 10, every frequency offset is set to an even-number multiple of the frequency amount R, which is defined by the information baud rate; and thereby, the reduction in the quality level of a transmission line is minimized.
As described above, according to the CFO-SS method, the individual communication stations use only the single pseudo-random code, thereby modulates the plurality of spread spectrum signal waves, and performs simultaneous transmission of the plurality of spread spectrum signal waves. This allows the limited frequency bands to be efficiently used, and in addition, allows a simply structured communication apparatus to implement high-speed transmission.
On the other hand, however, the method still arises the problems described below. When the spread spectrum signal wave separated by the frequency amount R exists at either one side or two sides of a desired spread spectrum signal wave, a quality level of transmission lines is reduced because of interference from other signal wave. To prevent the problem, every frequency offset is set to an even-number multiple of the frequency amount R defined by the information baud rate; and thereby, the reduction in the quality level of the transmission lines is minimized to allow the communication to be effected. This method, however, produces other problems. Since every frequency offset is set to an even-number multiple of the frequency amount R that is defined by the information baud rate, the essential information rate characteristics of the CFO-SS method cannot be made the best use. Specifically, compared to the case where every frequency offset is set to an integer multiple of the frequency amount R defined by the information baud rate, the information rate is reduced half.
An object of the present invention is to provide spread spectrum communication that allows a high-speed transmission to be implemented in a spread spectrum communication method and with a spread spectrum communication apparatus in which either a single or a plurality of communication stations use identical pseudo-random code series, the individual communication stations use different carrier frequencies differing from each other, and implement communication in a state where spectrums of signal waves thereby overlap with each other. In order to accomplish the above object, the present invention is characterized as follows. Among spread spectrum signal waves having carrier frequencies separated from each other in units of an integer multiple of the frequency amount that is defined by the information baud rate, all the spread spectrum signal waves having the carrier frequencies separated from each other in units of an even-number multiple of a frequency amount defined by the information baud rate are converged into a first group. Concurrently, all the spread spectrum signal waves that are adjacent to the aforementioned spread spectrum signal waves and that have the carrier frequencies separated in units of an even-number multiple of the frequency amount R are converged into a second group. Then, the first and the second groups of spread spectrum signal waves are simultaneously transmitted according to orthogonal polarized waves to perform information transmission. Thus, the individual communication stations simultaneously can transmit the maximum number of spread spectrum signal waves that can be theoretically obtained. Therefore, the present invention can provide a spread spectrum communication method and a spread spectrum communication apparatus that allows a high-speed transmission to be implemented.